Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Iijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes by evaporating carbon in an arc discharge. They reported carbon nanotubes having up to seven walls. In 1993, Iijima's group and an IBM team headed by Donald Bethune independently discovered that a single-wall nanotube could be made by vaporizing carbon together with a transition metal such as iron or cobalt in an arc generator (see Iijima et al. Nature 363:603 (1993); Bethune et al., Nature 363: 605 (1993) and U.S. Pat. No. 5,424,054). The original syntheses produced low yields of non-uniform nanotubes mixed with large amounts of soot and metal particles.
Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale.
Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have unique mechanical and electronic properties. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, number of concentric shells, and chirality of the tube.
Single-walled carbon nanotubes have been produced by simultaneously evaporating carbon and a small percentage of Group VIII transition metal from the anode of the arc discharge apparatus (Saito et al. Chem. Phys. Lett. 236: 419 (1995)). Further, the use of mixtures of transition metals has been shown to increase the yield of single-walled carbon nanotubes in the arc discharge apparatus. However, the yield of nanotubes is still low, the nanotubes can exhibit significant variations in structure and size (properties) between individual tubes in the mixture, and the nanotubes can be difficult to separate from the other reaction products. In a typical arc discharge process, a carbon anode loaded with catalyst material (typically a combination of metals such as nickel/cobalt, nickel/cobalt/iron, or nickel and transition element such as yttrium) is consumed in arc plasma. The catalyst and the carbon are vaporized and the single-walled carbon nanotubes are grown by the condensation of carbon onto the condensed liquid catalyst. Sulfur compounds such as iron sulfide, sulfur or hydrogen sulfides are typically used as catalyst promoter to maximize the yield of the product.
A typical laser ablation process for producing single-walled carbon nanotubes is disclosed by Andreas Thess et al. (1996). Metal catalyst particle such as nickel-cobalt alloy is mixed with graphite powder at a predetermined percentage, and the mixture is pressed to obtain a pellet. A laser beam is radiated to the pellet. The laser beam evaporates the carbon and the nickel-cobalt alloy, and the carbon vapor is condensed in the presence of the metal catalyst. Single-wall carbon nanotubes with different diameters are found in the condensation. However, the addition of a second laser to their process which give a pulse 50 nanoseconds after the pulse of the first laser favored the (10,10) chirality (a chain of 10 hexagons around the circumference of the nanotube). The product consisted of fibers approximately 10 to 20 nm in diameter and many micrometers long comprising randomly oriented single-wall nanotubes, each nanotube having a diameter of about 1.38 nm.
Many researchers consider chemical vapor deposition as the only viable approach to large scale production and for controllable synthesis of carbon single walled nanotubes (Dai et al. (Chem. Phys. Lett 260: 471 (1996), Hafner et al., Chem. Phys. Lett. 296: 195 (1998), Su. M., et al. Chem Phys. Lett., 322: 321 (2000)). Typically, the growth of carbon SWNTs by CVD method is conducting at the temperatures 550-1200° C. by decomposition of hydrocarbon gases (methane, ethylene, alcohol, . . . ) on metal nanoparticles (Fe, Ni, Co, . . . ) supported by oxide powders. The diameters of the single-walled carbon nanotubes vary from 0.7 nm to 3 nm. The synthesized single-walled carbon nanotubes are roughly aligned in bundles and woven together similarly to those obtained from laser vaporization or electric arc method. The use of metal catalysts comprising iron and at least one element chosen from Group V (V, Nb and Ta), VI (Cr, Mo and W), VII (Mn, Tc and Re) or the lanthanides has also been proposed (U.S. Pat. No. 5,707,916).
Presently, there are two types of chemical vapor deposition for the syntheses of single-walled carbon nanotubes that are distinguishable depending on the form of supplied catalyst. In one, the catalyst is embedded in porous material or supported on a substrate, placed at a fixed position of a furnace, and heated in a flow of hydrocarbon precursor gas. Cassell et al. (1999) J. Phys. Chem. B 103: 6484-6492 studied the effect of different catalysts and supports on the synthesis of bulk quantities of single-walled carbon nanotubes using methane as the carbon source in chemical vapor deposition. They systematically studied Fe(NO3)3 supported on Al2O3, Fe(SO4)3 supported on Al2O3, Fe/Ru supported on Al2O3, Fe/Mo supported on Al2O3, and Fe/Mo supported on Al2O3—SiO2 hybrid support. The bimetallic catalyst supported on the hybrid support material provided the highest yield of the nanotubes. Su et al. (2000) Chem. Phys. Lett. 322: 321-326 reported the use of a bimetal catalyst supported on an aluminum oxide aerogel to produce single-walled carbon nanotubes. They reported preparation of the nanotubes is greater than 200% the weight of the catalyst used. In comparison, similar catalyst supported on Al2O3 powder yields approximately 40% the weight of the starting catalyst. Thus, the use of the aerogel support improved the amount of nanotubes produced per unit weight of the catalyst by a factor of 5.
In the second type of carbon vapor deposition, the catalyst and the hydrocarbon precursor gas are fed into a furnace using the gas phase, followed by the catalytic reaction in a gas phase. The catalyst is usually in the form of a metalorganic. Nikolaev et al. (1999) Chem. Phys. Lett. 313: 91 disclose a high-pressure CO reaction (HiPCO) method in which carbon monoxide (CO) gas reacts with the metalorganic iron pentacarbonyl (Fe(CO)5) to form single-walled carbon nanotubes. It is claimed that 400 g of nanotubes can be synthesized per day. Chen et al. (1998) Appl. Phys. Lett. 72: 3282 employ benzene and the metalorganic ferrocene (Fe(C5H5)2) delivered using a hydrogen gas to synthesize single-walled carbon nanotubes. The disadvantage of this approach is that it is difficult to control particles sizes of the metal catalyst. The decomposition of the organometallic provides disordered carbon (not desired) the metal catalyst having variable particle size that results in nanotubes having a wide distribution of diameters and low yields.
In another method, the catalyst is introduced as a liquid pulse into the reactor. Ci et al. (2000) Carbon 38: 1933-1937 dissolve ferrocene in 100 mL of benzene along with a small amount of thiophene. The solution is injected into a vertical reactor in a hydrogen atmosphere. The technique requires that the temperature of bottom wall of the reactor had to be kept at between 205-230° C. to obtain straight carbon nanotubes. In the method of Ago et al. (2001) J. Phys. Chem. 105: 10453-10456, colloidal solution of cobalt:molybdenum (1:1) nanoparticles is prepared and injected into a vertically arranged furnace, along with 1% thiophene and toluene as the carbon source. Bundles of single-walled carbon nanotubes are synthesized. One of the disadvantages of this approach is the very low yield of the nanotubes produced.
It is generally recognized that smaller catalyst particles of less than 3 nm are preferred for the growth of smaller diameter carbon nanotubes. However, the smaller catalyst particles easily aggregate at the higher temperatures required for the synthesis of carbon nanotubes. U.S. Patent Application No. 2004/0005269 to Huang et al. discloses a mixture of catalysts containing at least one element from Fe, Co, and Ni, and at least one supporting element from the lanthanides. The lanthanides are said to decrease the melting point of the catalyst by forming alloys so that the carbon nanostructures can be grown at lower temperatures.
Aside from the size of the catalyst, the temperature of the reaction chamber can also be important for the growth of carbon nanotubes. U.S. Pat. No. 6,764,874 to Zhang et al. discloses a method of preparing nanotubes by melting aluminum to form an alumina support and melting a thin nickel film to form nickel nanoparticles on the alumina support. The catalyst is then used in a reaction chamber at less than 850° C. U.S. Pat. No. 6,401,526, and U.S. Patent Application Publication No. 2002/00178846, both to Dai et al., disclose a method of forming nanotubes for atomic force microscopy. A portion of the support structure is coated with a liquid phase precursor material that contains a metal-containing salt and a long-chain molecular compound dissolved in a solvent. The carbon nanotubes are made at a temperature of 850° C.
Thus, it is well known that the diameter of the SWNTs produced is proportional to the size of the catalyst particle. In order to synthesize nanotubes of small diameter, it s necessary to have catalyst particles of very small particle size (less than about 1 rim). Catalysts of small particle size are difficult to synthesize, and even with small catalyst particle sizes, a distribution of catalyst sizes is obtained which results in the formation of nanotubes with a range of diameters.
One solution to the synthesis of uniform diameter nanotubes is to use a template, such as molecular sieves, that have a pore structure which is used to control the distribution of catalyst size and thereby the size of the SWNTs formed. Thus, the diameter of SWNT can be changed by changing the pore size of the template. These methods are not versatile. Thus, there is a need for methods and processes for controllable synthesis of carbon single walled nanotubes with small and narrow distributed diameters. Accordingly, the present invention provides novel methods and processes for the synthesis of SWNTs with small and narrow distributed diameters.